A heat pump system generally comprises a compressor unit and a heat exchange system including a relatively low temperature evaporator and a relatively high temperature condenser. Frequently, the compressor and the evaporator heat exchanger are combined in a unit, the compressor being generally disposed interiorly of the cooling coils of the evaporator so that air cooled by the evaporator can be used to cool the compressor. Recent studies have shown that for a variety of reasons such an arrangement is wasteful of energy. As a result, it has been suggested to separate the compressor from the evaporator, installing the former indoors and the latter outdoors to minimize heat losses during cold weather.
During operation of the heat pump, there is frequently a buildup of frost or ice on the heat exchanger coils of the evaporator. The amount of ice that forms is a function of the ambient temperature and the relative humidity, the buildup being greatest at temperatures around 0.degree. C. and at a saturated humidity. Since the frost buildup impedes the airflow past the heat exchanger, the unit must be defrosted periodically. In the past, the defrosting cycle was normally initiated at predetermined time intervals. Of course, the time intervals had to be chosen to effect adequate defrosting when the frost buildup is at a maximum. Consequently, during other temperature and humidity conditions, when the frost buildup is less the system defrosts too frequently. This resulted in a significant waste of energy.
Attempts have also been made in the past to initiate defrost cycles by sensing the change in the pressure drop across the heat exchanger. With a buildup of frost the pressure on the downstream side of the heat exchanger decreases. This decrease is sensed with a differential barometric pressure switch. The difficulty with such an arrangement is that the differential pressure switch must close contacts reliably on pressure changes in the order of as little as 1/50th inch W.G. Up to the present it has been difficult or impossible to make such sensitive pressure switches, at least on an economically feasible basis.
Aside from problems of determining the initiation (and termination) of defrost cycles, prior art heat pump systems were less than fully satisfactory as far as the performance of the actual defrost cycle is concerned. Normally, prior art heat exchangers are constructed so that air flows through them along an essentially horizontal path. Unless previously heated air is uniformly heated and well mixed, there are significant temperature differentials between the upper and lower portion of the horizontal path leading to unequal defrost times, local overheating, and the like, all of which is undesirable. As a consequence, defrosting systems have commonly been chosen which heat the frosted coil internally by reversing the refrigeration cycle. Such operation, with a warm evaporator and cold condenser, generates abrupt pressure reductions in the system low side, a low refrigerant flow and a high probability of inefficient cooling and lubrication of the hermetically sealed motor-compressor and, therefore, an increased compressor failure rate.